This invention relates generally to electronic oscillators, and in particular, to an oscillator having an active feedback resistance circuit to set the desired operating condition of the oscillator""s amplifier.
Current microprocessor systems use a crystal oscillator as part of a Real Time Clock (RTC) to keep track of the date and time of day. They typically use a relatively accurate and high frequency crystal oscillator, for example 32.768 kHz, which is divided to generate seconds, minutes and hours for the system. Because the Real Time Clock (RTC) needs to be running even when the microprocessor system is off, it is directly connected to a battery. The dependency on battery power raises power consumption issues among other accuracy, stability and manufacturing issues, as will be discussed with regard to the following example.
FIG. 1 illustrates a block diagram of a prior art processor system 100. The processor system 100 consists of a microprocessor 108 coupled to a memory controller 106, which is sometimes referred to in the relevant art as the xe2x80x9cnorth-bridge.xe2x80x9d The memory controller 106 interfaces with the system memory 110. The processor system 100 further consists of an input/output (I/O) bus 102 coupled to an I/O controller 104, which is sometimes referred to as the xe2x80x9csouth-bridge.xe2x80x9d The xe2x80x9csouth bridgexe2x80x9d is, in turn, coupled to the xe2x80x9cnorth bridge.xe2x80x9d Typically included in the xe2x80x9csouth-bridgexe2x80x9d circuit board is the Real Time Clock (RTC) for the processor system 100, which keeps track of the time and date for the system.
For discussion purposes, FIG. 1 only shows the crystal oscillator 120 portion of the Real Time Clock (RTC) for the processor system 100. The crystal oscillator 120 consists of an amplifier 122 including a crystal resonator 124, an external resistor 126, and a pair of capacitors CL11 and CL22. The crystal resonator 124 and external resistor 126 are connected between the input and output of the amplifier 122, i.e. in feedback with the amplifier. The capacitor CL11 is coupled between the input of the amplifier 122 and ground potential. Similarly, the capacitor CL12 is coupled between the output of the amplifier 122 and ground potential. The crystal resonator 124 resonates precisely at a particular frequency, which causes the oscillator 120 to generate a periodic signal cycling at such frequency. The external resistor 126 biases the amplifier 122 which affects its gain. The capacitors CL11 and CL12 serve to optimize the startup and loading conditions of the oscillator 120.
There are several drawbacks with regard to the external resistor 126 of the prior art oscillator 120. One set of drawbacks arises from the fact that the external resistor 126 sets the gain of the oscillator 120. In order to satisfy the condition for oscillation, the gain of the amplifier 122 should be at least one (1). However, a gain significantly over one (1) could lead to additional noise in the output signal of the oscillator 120, could also lead to instability of the oscillator 120, and could unduly increase the power consumption of the oscillator 120. Thus, the external resistor 126 should be precisely selected such that the gain of the amplifier 122 is slightly above unity gain. Because there are process variations with regard to the integrated circuit in which the amplifier 122 is formed, there can be substantial trial and error in selecting an external resistor 126 that sets the gain of the amplifier 122 slightly above unity. Such trial and error increases the costs, time and complexity of manufacturing the oscillator 120 in addition to reducing the reliability of the oscillator 120. In addition, once the external resistor 126 is selected, it becomes impractical to change the resistor later on to account for changes in the oscillator""s performance due to aging or other changes in the environment and/or application.
Another set of drawbacks stems from the fact that the external resistor 126 lies external to the integrated circuit in which the amplifier 122 is formed. Since the external resistor 126 lies external to the amplifier integrated circuit, it is typically mounted on the xe2x80x9csouth bridgexe2x80x9d circuit board along with the integrated circuit. This increases the board routing complexity as well as the manufacturing of the xe2x80x9csouth bridgexe2x80x9d board, which leads to increased manufacturing time and costs. Also, because the external resistor 126 is situated external to the shielded integrated circuit, it is exposed to environment noise, thereby introducing additional noise into the oscillator signal.